Anti-depressant medication can affect a patient’s brain in just three hours, according to a new study.

Scientists tested the widely-prescribed drug escitalopram - marketed as Cipralex in the UK, or Lexapro in the US - which contains a serotonin re-uptake inhibitor (SSRI).

Seratonin is a neurotransmitter, a type of messenger chemical, that carries signals between nerve cells in the brain. SSRIs work by allowing more serotonin to pass messages between nearby nerve cells, according to the NHS.

It was previously understood that SSRIs affect a patient over a period of weeks, but a study by scientists from the Max Planck Institute for Human Cognitive and Brain Sciences in Germany published in the journal ‘Current Biology’ shows it could take as little as three hours.

To make their findings, scientists scanned the brains of volunteers who had taken escitalopram to measure their blood flow.

The results showed that the drug reduced connectivity in the majority of the brain - with an increase in brain connectivity in the cerebellum and thalamus regions.

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"We were not expecting the SSRI to have such a prominent effect on such a short time scale or for the resulting signal to encompass the entire brain," said study leader Dr Julia Sacher, from the Max Planck Institute for Human Cognitive and Brain Sciences in Germany, Mail Online reported.

While doctors widely prescribe SSRIs, it is still not entirely clear how they work.

Dr Sacher said: "What we are seeing in medication-free individuals who had never taken antidepressants before may be an early marker of brain reorganisation.”

She added the new findings could be an essential first step toward clinical studies in patients suffering from depression. Researchers now hope to compare the brains of recovering patients, and those who fail to respond to SSRI treatment.

Dr Sacher said: "Understanding the differences between the brains of individuals who respond to SSRIs and those who don't could help to better predict who will benefit from this kind of antidepressant versus some other form of therapy.

"We hope our work will help to guide better treatment decisions and tailor individualised therapy for patients suffering from depression."